Process plant elements, such as valves, typically have associated mounted instruments, such as a valve position controller and/or a position transmitter, that control the elements and/or transmit information about the element to implement one or more desired process(es) and/or operation(s) within a process plant. An example valve assembly includes a diaphragm-type or piston-type pneumatic actuator, which is controlled by an electro-pneumatic valve position controller. Example electro-pneumatic valve position controllers receive one or more control signals (e.g., a 4-20 milliamps (mA) control signal, a 0-10 volts direct current (VDC) control signal, a digital control signal, etc.), and convert the control signal(s) into one or more pneumatic pressures that are provided to the pneumatic actuator to open, close or hold a position of a corresponding valve. For example, if a process control routine determines that a pneumatically-actuated normally-closed stroke-type valve is to permit the passage of a greater volume and/or rate of flow of a process fluid, the magnitude of the control signal supplied to the electro-pneumatic valve position controller associated with the valve may be increased from 4 mA to 8 mA, assuming the use of a current type of control signal.
In some examples, the electro-pneumatic valve position controller uses a feedback signal generated via a feedback sensing system or element, such as a position sensor. Such feedback signals represent the position of the pneumatic actuator and the corresponding valve. The valve position controller compares the feedback signal to a control signal representing a desired set-point or desired valve position (e.g., 35% open), and determines whether to adjust one or more of the pneumatic pressures provided to the actuator. For the valve position controller, the actuator and the valve combination to operate as intended within the process plant, the valve position controller may need to be calibrated to the feedback-sensing element.